A May 10, 2005 report by web analytics company WebSideStory estimates usage in the United States of Microsoft's Internet Explorer web browser to have fallen to 88.9% by April 29, 2005, from 89.9% on February 18, 2005. Microsoft's strongest competitor, Mozilla Firefox, improved its market share from 5.7% to 6.8%.

All browsers based on Mozilla program code combined have a market share of 9%, while other browser alternatives combined make up a share of 2.1%. The report also contains data from Germany and Japan. Firefox was found to have a much higher usage share in Germany (22.6%), and a lower one in Japan (2.8%), where Internet Explorer still holds 93.9% of the market. Browser usage also varies strongly for different demographics.

In most countries, Microsoft's Internet Explorer has long enjoyed a market share above 90 percent. Many have attributed this to the browser's deep integration into the Microsoft Windows operating system. While Firefox market share is still increasing in the United States, its growth has slowed down compared to a very rapid increase after its widely reported 1.0 release on November 9, 2004. Technology writer Paul Festa speculated in an editorial for Silicon.com that the decreasing growth rate "could mean that the browser has converted a substantial proportion of its natural constituency, thought to be early adopters and the technically savvy."

Firefox is a free web browser developed by the non-profit Mozilla Foundation in collaboration with volunteer software developers. The program is open source software, that is, the underlying program code is available free of charge for anyone to download, modify and distribute. Firefox commands a well-organized community of fans who promote the browser on their websites, blogs, and by other means. They tout the browser as more functional and secure than Internet Explorer; the latter argument has been somewhat weakened by recently discovered critical security flaws.

Some corporations are also making steps towards promoting Firefox as an alternative to Internet Explorer. CNET News reported on Thursday that technology giant IBM is encouraging its employees to use Firefox. IBM is also promoting other open source technologies such as the free Linux operating system.